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M-shape Characteristic shape shown in an analysis profile of nickel distribution in Widmanstätten structure in meteorites. The temperature-dependence of the diffusion of nickel within the metal phase away from kamacite (Ni—Fe alloy with 6% Ni) into taenite (Ni—Fe alloy with 30% Ni), and the faster rate of diffusion in kamacite, leads to disequilibrium nickel distribution and the M-shape.